SSZT308 may 2020 ISO1042-Q1 , SN6505B-Q1
This article appeared in Electronic Designand has been published here with permission
Designing for today’s automobiles is a balancing act.
Between meeting increasingly strict emissions standards and powering the growing number of on-board systems and gadgets, today’s vehicles need to be as efficient as they are high powered. To achieve this convergence of efficiency and power, engineers are relying more and more on systems that combine 48-V electric power operation with conventional gas engines. This approach ensures that vehicles meet strict carbon-di-oxide (CO2) emissions while also improving performance and drive quality.
While much has been said about dual-battery automotive systems themselves, I would like to focus on a critical and sometimes overlooked component in these combined 12- and 48-V systems: galvanic isolation. Galvanic Isolation is used for immunity against ground noise and to protect 12-V systems in case of ground lifts or fault conditions in the 48-V systems to which they connect.
In this article, I will discuss the need for isolation in 48-V automotive applications, and describe a compact, efficient, robust and low-noise method for isolating 48-V systems through the Control Area Network (CAN) interface.
Keeping in mind the above scenarios, the Verband der Automobilindustrie 320 (VDA320) standard for electric and electronic components in motor vehicles specifies a degree of isolation between the 48-V and 12-V domains. A dielectric withstand test is performed, where a 60-V test voltage is applied across the 48-V to the 12-V barrier for 60 mins, where the system must offer a 1 M-Ω impedance, and not be damaged by the test. A fault current test is also performed, where a 70-V is applied across the barrier, and the resultant current flowing must be less than 1 µA. The use of galvanic isolation, as shown in Figure 1, enables compliance to these requirements.
An isolated DC/DC converter derives an isolated power supply, VISO, that powers parts of the 48-V system. VISO ensures that the digital isolator and critical parts of the 48-V system have power available for operation, even if the 48-V battery is fully discharged. VISO can also be used to put the 48-V side into a safe state in case of disconnection of GND_48V.
New integrated isolated CAN transceivers and isolated DC-DC power supply controllers are now available that help simplify the isolated CAN interface in 48-V systems. Figure 3 shows an example 48-V starter generator. You can use similar isolation architectures for other 48-V systems, such as DC/DC converters, battery-management systems, heaters and air compressors.
Single-chip integrated isolated CAN transceivers such as the ISO1042-Q1, shown in Figure 2, integrate high-voltage galvanic isolation with a high-performance CAN transceiver to help reduce board area while improving timing parameters. From the CAN point of view, low loop delays and skews enable high-speed data communication using CAN Flexible Data-Rate. Isolation provides immunity against conducted and radiated disturbances. Redundant or reinforced isolation can provide extra margin for protection under fault conditions.
When used with an external transformer, a push-pull transformer driver such as the SN6505B-Q1, also shown in Figure 2, can generate an isolated power supply, VISO_HV, (in the range of 10 V to 15 V) to power metal-oxide semiconductor field-effect transistor gate drivers, and a lower VISO (in the range of 3.3 V to 5 V) to power the microcontroller and digital side of the isolated CAN device. The push-pull topology uses two low-side switches that are on in alternate clock phases to transfer power continuously across a center-tapped isolation transformer. The topology uses feed-forward regulation, with the output voltage controlled purely through transformer ratios. Continuous power transfer results in much lower peak currents compared to other topologies, resulting in lower emissions and higher efficiency. The symmetric drive also prevents transformer saturation, resulting in compact transformers.
On the 12-V side, a non-isolated DC/DC converter or buck can generate a 5-V supply to supply the CAN transceiver, while also serving as the input voltage for the push-pull isolated DC/DC converter. Using a buck upfront makes the system insensitive to variations in the 12-V battery power supply that can be caused by load changes. Also, operating with a lower input voltage (5 V vs. 12 V) results in smaller transformers.